1. Field of the Invention
This invention relates to a method and apparatus for converting an analog video signal, which has been read from a magnetic recording medium such as a floppy disk or magnetic tape, into digital image data and writing the digital image data in a memory.
2. Description of the Related Art
In order that digital image data produced by A/D-converting an analog video signal read from a magnetic recording medium may be written in a memory while being correctly synchronized to the video signal, a horizontal synchronizing signal is separated and extracted from the video signal and the generation of a timing clock signal or the like for the A/D conversion is synchronized to the horizontal synchronizing signal. More specifically, the horizontal synchronizing signal separated and extracted from the video signal is applied to an automatic frequency controller (also referred to as an "AFC" circuit) including a phase-locked loop (referred to as a "PLL"), a trigger signal tuned to the horizontal synchronizing signal is obtained from the AFC circuit, and the circuit which generates the timing clock signal is reset by the trigger signal. By being reset, the circuit for generating the timing clock signal is returned to the initial state and again generates a series of timing clock signals.
In general, an analog video signal is recorded on a floppy disk, magnetic tape or the like on a field-by-field (or frame-by-frame) basis, and a point of discontinuity is present in the video signal at the boundaries between fields (or frames). The point of discontinuity is in the vicinity of a vertical synchronizing signal, and the point of discontinuity is where the horizontal and vertical synchronizing signals tend to pick up noise. If this noise is applied to the AFC circuit, the latter will more readily generate an erroneous trigger signal as the accuracy of its frequency-control capability (i..e., its capability to follow up an input signal becomes higher). If the circuit which generates the timing clock signal is reset at a point in time other than that at which the horizontal synchronizing signal appears, the writing of the digital image data in the memory will no longer be synchronized to the original analog video signal and the position at which the image data is written in the memory will be adversely affected. If this occurs, a distorted image, for example, will appear when the image data is read out of the memory and reproduced.
In a case where recording by field is performed on a floppy disk (i.e., a case where one frame of an image is recorded in one field), field/frame conversion processing is executed in order to produce a pseudo-frame signal when the recorded image is played back. The video signal read from the floppy disk in field/frame conversion processing is delayed by 0.5 H (where 1 H is the horizontal scanning interval) every other 1 V (the vertical scanning interval). The horizontal synchronizing signal tends to be lost when a changeover is made between the signal delayed by 0.5 H and a through signal (a signal which is allowed to pass without being delayed). As in the case where noise is picked up as mentioned above, the writing of the image data in the memory can no longer be synchronized to the original video signal if the horizontal synchronizing signal is lost.